Steve Pavlina has written a very good article on getting up early.

He says:

……The solution was to go to bed when I’m sleepy (and only when I’m sleepy) and get up with an alarm clock at a fixed time (7 days per week). So I always get up at the same time (in my case 5am), but I go to bed at different times every night……

The original is very clear straight forward. Steve also has an updated Part II where he details some more about the approach. All in all very good read.